Process for preparing low molecular carboxylic acid anhydrides



Feb. 3, 1959 w. VOGT 2,872,481

PROCESS FOR PREPARING LOW-MOLECULAR CARBOXYLIC ACID ANHYDRIDES FiledNov. 7, 1955 A 4 iner'f gas phase rich in wa ter mixture containingmhydp'de INVENTO R WILHELM I/OGT WATToRNEYs trite PROCESS FOR PREPARINGLOW MOLECULAR 'CARBOXYLI-C ACll) ANHYDRIDES Wilhelm Vogt, Knapsack, nearKoln, Germany, assignor to Knapsack-Griesheim Aktiengesellschaft,Knapsack, near Koln, Germany, a corporation of Germany ApplicationNovember 7, 1955, Serial No. 545,477

Claims priority, application Germany November 13, 1954 6 Claims. (Cl.260-546) Now I have found that monobasic saturated aliphatic carboxylicacids with 2 to 4 carbon atoms, can be dehydrated in a simple manner toyield the corresponding 7 anhydrides without intermediate formation ofketene, by heating the carboxylic acids in the liquid phase under thevapor pressure of the substances present in the reaction mixture, i. e.under a pressure of about 2 to about 50 atmospheres (gauge pressure),preferably about 10 to about 30 atmospheres (gauge pressure), and attemperatures ranging from about 180 C. to the critical temperature,suitably to about 270 C., preferably from about 200 C. to about 250 C.The upper limit of the superatmospheric pressure applied is determinedby the vapor pressure of the corresponding carboxylic acids and of thatof an entrainer, if used, which vapor pressures correspond to thecritical temperature of these compounds or a temperature of about 270 C.The water which has been formed is suitably distilled off in admixturewith carboxylic acid at the head of a distilling column, while thecarboxylic acid anhydride, diluted with non-reacted carboxylic acid,remains in the distilling vessel.

To the carboxylic acid there may advantageously be added the knownagents for entraining the water formed, especially aliphatic, aromaticor hydroaromatic hydrocarbons such as benzene, n-hexane or cyclohexaneor, if desired, carboxylic acid esters which, under the conditionsapplied, are still sufiiciently stable, for example those obtained fromacids and alcohols containing up to carbon atoms each, such asethylacetate, propylacetate, butylacetate, propylpropionate,ethylpropionate, methylpropionate, methylbutyrate, ethylbutyrate orpropylbutyrate. Such entraining agents should be used as form azeotropicmixtures with water, are immiscible with Water and suitably have aboiling point at least 20 C. below that of the acid to be obtained, sothat they are present at the head of the column in a high concentration.Furthermore,,the condensate of the azeotropic mixture should possiblyseparate, at least in certain temperature ranges, into two phases ofwhich only the one rich in water is drawn otf continuously, while theother one poor in water is partly or, advantageously, totally returnedinto the column.

In the preparation of acetic anhydride, for example,

theprocess of the invention is suitably carried out at a Patent 0temperature within the range of C. and approximately the criticaltemperature of the acetic acid or the entraining agents, advantageouslybelow about 270 C., the pressure being 5 to 50 atmospheres (gaugepressure) corresponding to the vapor pressures of the components. Theprocess is accompanied by little decomposition only, particularly at lowand medium temperatures--provided the walls of the apparatus are made ofsuch a material as has no or only a small catalytic eifect on thedecomposition of the organic materials, for example copper, enamelediron, silver, tantalum or, if desired CnNi-Mo steeland therefore leadsto high yields of anhydride.

Since the reaction velocity is sufficiently high at tem-' peratures fromabout 180 C. to about 270 C., dehydration catalysts can be dispensedwith; they may, however, be concomitantly used in some cases.

An apparatus suitable for use in carrying out the process of theinvention is illustrated diagrammatically by way of example in theaccompanying drawing:

The discontinuous distillation can be carried out in a small copperapparatus consisting of a still 1 and a column tube 2 filled withpacking bodies and suitably well protected against loss of heat. Fromcondenser 3, arranged above the tube, the condensate runs ofi and isreturned into the column by pipe 4. In this pipe a separator 5 isinstalled from which in the case of disstillations carried out withentrainers the phase rich in water, 'for example the phase having agreater specific weight, can be drawn olf. The distilling apparatus isconnected with a pressure vessel 6 which has about 10 times the volumeof the apparatus and can be filled, before being operated, with acompressed inert gas, for example nitrogen, to about working pressure.

Depending on the working conditions used in the discontinuousdistillation carried out with or without an entrainer, the formation ofanhydride practically stops when a certain conversion has been attained.No anhydride is formed at all when already before the distillation sucha quantity of anhydride is added to the distillation mixture as would beformed as a maximum after a prolonged time of distillation under thesame conditions but without addition of an anhydride.

When the conversion of, for example, acetic acid into acetic anhydrideis plotted against the time the first part of the curve rises inapproximately a straight line. The slope of the curve graduallydecreases. When using benzene as entrainer and proceeding at 265 C.under a pressure of 30 atmospheres (gauge), the extent of conversionamounts to about 5 percent after 3 hours of distillation and to about8.4 percent after 10 hours of distillation under the same conditions.When operating at 210 C. under a pressure of 10 atmospheres (gauge-'pressure) the extent of conversion amounts to about 3.4

percent after 6 hours. With the use of n-heXane as entrainer, theconversion amounts to about 6 percent after heating at 260 C. for 8hours under a pressure of 27 atmospheres (gauge pressure).

The above described gradually decreasing slope of the curve becomesunderstandable if the total dehydration process is assumed to be dividedinto .two stages. The process of the first stage of the chemicalconversion of acetic acid into anhydride and water stops when in thesecond stage, i. e. during the physical accumulation of water in thedistilling column, for example by separation of a phase rich in water inthe condensate, the quantity of water removed from the column is notgreater than thatreturned to the column by the phase poor in water. In acase of a distillation without an entrainer, the conversion ispractically finished when the reaction water is present at the head ofthe column in such a concentration that it would be necessary to. carryalong all the acid present in the apparatus in order to remove thewater. As results from this, the conversion into anhydride depends onthe efiicacy of the column so that columns of the highest possibleefiiciency must be used in order to obtain higher yields. of' anhydride.

When. proceeding under such conditions that no condensate consisting oftwo. phases is formed, for example when operating without an entrainingagent, the hydrous product distilling off. at the head of. the pressurecolumn can be freed from water by' known: methods in a seconddistillation column under normal pressure. The re sultant anhydrouscarboxylic acid can be returned into the pressure column by means of apump. In'a continuous process the anhydride-containing product (I)runningv off. from the distillation: vesseliof the pressure column can.be separated into carboxylic acid and an: hydride under normal pressurein a third distillation column. The carboxylic acid. separated during;this pro= cedure can also be returned into the pressure column and, ifdesired, the anhydride can be distilled once more. The quantities ofheat necessary'for heating: the second and third column may be providedfor the'major part by the heat set free, at thehigh working temperatureof the pressure column, in the condenser of the latter and from the heatcontained in the product I.

The process described above is particularly suitable for producingacetic anhydride. It may, however, be used in the same manner forobtaining other anhydrides of fatty acidsof low molecular weight suchaspropionic.

or butyric acid anhydride. To this end thereaction is carried outwithin. the range of temperature applied in the manufacture of aceticanhydride andgenerallytakes a more rapid course than the preparation ofacetic anhydride. The working pressure is correspondingly lower. Whenstarting from an acid. mixture, mixtures'of anhydride can also beobtained. Uniform anhydrides are; of course, more desired in industry.

The following examples serve to illustrate the invention, they are,however, not intended to limit it thereto. They are chiefly concernedwith the discontinuous mode of, executing the'process according to theinvention. It should be mentioned that by returning the recoveredcarboxylic acids and entrain'ing agents according to-the continuousmodeof executing the process of the invention described abovesubstantially the total amount of carboxylic acid used can beconverted'into the corresponding anhydride.

Example 1 In a distilling apparatus made of copper and resistant tocompression and which consists of a still having a capacity of 3 litresand a packed column, 2' metres long and 25 millimetres in diameter,containing filling bodies, a mixture of 1000 grams of acetic acid and800 grams of benzene is boiled under a pressure of nitrogen ofatmospheres (gauge pressure), corresponding to a boiling temperature of210 C., and the volatile constituents consisting of acetic acid, waterand benzene are distilled off. By cooling to about 5 C., a condensate isformed which seperates into two phases, of which the one having thelower specific gravity consists chiefly of benzene, in addition to jpercent of acetic acid, while the phase having the higher formed duringthe formation of anhydride. The lower phase which is rich in water isdrawn off continuously. After distilling for 6 hours and returning, per

specific gravity contains in addition to 66 per ent of acetic acid,the'water' action water is nearly completed.

l- Example 2 Example 3 in: a manner analogous to that described inExample 1 a mixture of acetic acid and benzene of the same compositionas in Example 1 is boiled at 265 C. under pressure of inert gas of 30atmospheres (gauge pressure). After 10 hours 8.4- percent of the aceticacid used has been converted into anhydride. In an apparatus constructedfor use on a half-technical scale, the acetic acid and the benzene-arereturned in the original relative proportions into the pressure columns,after the acetic acid and the benzene have been separated from the waterin known? manner inv additional distillation columns under ordinarypressure. In this manner substantially nearly all the acetic. acid usedcan be gradually converted into theanhydride by being repeatedlyrecycled and by further additions of acetic acid and benzene.

Example 4 in a manner analogous to that described in Example 1 a mixtureof 1000 grams of acetic acid and 600 grams of n-hexane is boiled at 212C. under pressure of nitrogen of'1'2 atmospheres (gauge pressure) thestarting material being. returned to the still. After 12 hours ofdistillation the separation of the second phase of the condensate whichphase. has the higher specific gravity and contains about 68. percent ofacetic acid in addition to water, takes only a slow course. After 20hours 5.2 percent of the acetic acid. used has been converted intoacetic anhydride.

Example 5 The mixture of acetic acid and n-hexane described in Example 4is boiled at 260 C. under a gas pressure of 27 atmospheres (gaugepressure) the starting material being returned to the still. After 20hours, the extent of conversion into acetic anhydride amount to 7.8percent. By-returning'the acetic acid and the n-hexane to the reactionprocess' in the: manner described at the end of Example 3, almost thewhole amount of the acetic acid used can be converted into anhydride.

Example 6 In'the column described in Example 1 a mixture of 1000 gramsof acetic acid and 600 grams of cyclohexane is distilled at 250 C. undera gas pressure of 17 atmospheres (gauge pressure); the speed ofrecycling being 3 litres per hour. The condensate remains one-phased andabout percent of it is acetic acid. Within the course 0134 hours 1100grams of distillate pass over during which procedure-3 percent of theacetic acid used is converted into acetic anhydride.

Example 7 In the column described in Example 1', 1900 grams of aceticacid are distilled at 230 C. under a pressure of inert gas of 10atmospheres (gauge pressure) without addition of an entrainer. The speedof recycling amounts to 3 litres per hour. About 1200 grams ofdistillate containing; the reaction water are-obtained Within the courseoff; hours. 2.2 percent of the acetic acid use is converted into: aceticanhydride.

Example 8 As'described in Example 7 in the course of 4 hours a fractionof 1000 grams is distilled oti from 1900 grams of weakly hydrous aceticacid at a boiling temperature of 250 C. under a pressure of inert gas of17 atmospheres (gauge pressure) without the addition of an entrainer,

the speed of recycling amounting to 3 litres per hour. In addition tothe Water content of the starting material, the said fraction contains17 grams of reaction water. 4 percent of the acetic acid used has beenconverted into acetic anhydride.

Example 9 In the column described in Example 1, 1000 grams are distilledofi from 1800 grams of propionic acid, without addition of an entrainerat 235 C., under a pressure of inert gas of 8 atmospheres within thecourse of 2 hours. The speed of recycling amounts to 3 litres per hour.6.7 percent of the propionic acid used is converted into propionic acidanhydride which remains in the still mixed with the starting product. Itcan be easily obtained in a pure form from the content of the still byfractional distillation under reduced pressure. By returning thepropionic acid to the reaction process in the manner described at theend of Example 3, more than 90 percent of the propionic acid used isconverted into anhydride.

Example 10 In the column described in Example 1, 1800 grams of butyricacid are distilled at 230 C. under a pressure of nitrogen of 3.5atmospheres (gauge pressure). 7 The speed of recycling amounts to 2litres per hour. Within 1 hour, 3.4 percent of the butyric acid isconverted into anhydride after 1400 grams of distillate have passedover. The resulting anhydride can be obtained easily in a pure form fromthe content of the still by distillation under reduced pressure. In thiscase, too, the major part of the butyric acid can be converted into theanhydride in the manner described at the end of Example 3.

I claim:

1. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atomsunder a pressure of about 2 to about 50 atmospheres and at a temperatureof between about 180 C. and about 270 C. and distilling the water formedfrom the reaction mixture.

2. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atomsunder a pressure of about 2 to about 50 atmospheres and at a temperatureof between about 180 C. and about 270 C. in the presence of a waterentrainer for the water formed in the process and distilling the waterand water entrainer mixture from the reaction mixture.

3. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atomsunder a pressure of about 2 to about atmospheres and at a temperature ofbetween about C. and about 270 C. in the presence of a hydrocarbon waterentrainer for entraining the water formed in the process, distilling thewater and water entrainer mixture from the reaction mixture andreconducting Water entrainer and acid, distilled over with the water,into the reaction zone.

4. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms ata pressure of about 2 to 50 atmospheres and at a temperature of betweenabout 200 C. and about 250 C. in the presence of a hydrocarbon waterentrainer, for entraining the water formed in the process, distillingthe water and water-entrainer mixture from the reaction mixture andreconducting water entrainer and acid distilled over with the water,into the reaction zone, the pressure resulting from the vapour pressuresof the components used in the process.

5. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atomsunder a pressure of about 2 to about 50 atmospheres and at a temperatureof between about 180 C. and about 270 C., distiiling the water formedfrom the reaction mixture and reconducting acid, distilled over with thewater, into the reaction zone.

6. A process of preparing acid anhydrides which comprises heatingmonobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms ata pressure of about 2 to 50 atmospheres and at temperatures betweenabout 200 C. and about 250 C., distilling the water formed from thereaction mixture and reconducting acid, distilled over with the water,into the reaction zone, the pressure resulting from the vapor pressuresused in the components of the process.

Dreyfus Nov. 19, 1929 Schleichler et al July 12, 1931

1. A PROCEEE OF PREPARING ACID ANHYDRIDES WHICH COMPRISES HEATINGMONOBASIC SATURATED ALIPHATIC CARBOXYLIC ACIDS OF 2 TO 4 CARBON ATOMSUNDER A PRESSURE OF ABOUT 2 TO ABOUT 50 ATMOSPHERES AND AT A TEMPERATUREOF BETWEEN ABOUT 180* C. AND ABOUT 270* C. AND LISTILLING THE WATERFORMED FROM THE REACTION MIXTURE.